1. Field of the Invention
The present invention relates generally to semiconductor wafer planarizing, and more particularly, to methods and systems for controlling and mixing chemicals for a chemical mechanical planarizing process.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform a variety of substrate preparation and fabrication operations including chemical mechanical planarization (CMP) operations, substrate cleaning, substrate polishing and buffing, substrate rinsing and drying, and other similar operations. Planarization, polishing, and cleaning operations are routinely performed on semiconductor wafers at various stages in the fabrication process. Typically, such operations are efficiently combined within process systems that are configured, for example, to receive batches of wafers at a time to be processed through CMP, polishing, buffing, cleaning, rinsing, and/or drying, followed by wafer processing through subsequent wafer fabrication operations.
Typically the chemicals required for such a CMP processes are prepared in a batch process system 100 such as shown in FIG. 1. FIG. 1 is a schematic diagram of a prior art system for mixing chemicals for a CMP process. A first chemical 101 is stored in a first supply tank 102 and a second chemical 103 is stored in a second supply tank 104. When a batch of the first and second chemicals 101, 103, is mixed, the respective supply valves 106, 108 are opened and a selected amount of the first and second chemicals 101, 103 are transferred to the batch mixing tank 110. The first and second chemicals 101, 103 are then mixed in the batch-mixing tank 110. Typically the mixed batch is tested through manual processes such as weighing the respective quantities of the first and second chemicals 101, 103 that are added to the batch mixing tank 110. Once the mixed batch of the chemicals is fully prepared and ready to be used, the batch supply valve 120 is opened and the batch-mixing tank 110 is pressurized to cause the mixture 123 to flow to a delivery tank 122. The delivery tank 122 can then be pressurized to deliver the mixture 123 to a mixture distribution manifold 124. The manifold 124 distributes the mixture to multiple points of use 130, 132, 134, through point of use supply valves 136, 138, 140 respectively. Each of the points of use 130, 132, 134 can represent a different CMP process tool or different locations within a single CMP process tool.
One of the problems with the batch process system 100 described above is that often the mixture 123 can only be used for a limited time period. For example, often, optimum CMP results require the mixture be used within the first sixty minutes after the mixture 123 is formed in the batch-mixing tank 110. The time limits may be due to reactivity of the mixture 123 or due to coagulation effects common to the slurry-type chemical used in the CMP process.
Another problem with the batch process system 100 is that the mixture 123 must be transferred to each point of use 130, 132, 134 via a distribution system (e.g., the manifold 124, the respective point of use supply valves 136, 138, 140 and interconnecting piping). When each batch of the mixture 123 expires or is no longer needed, the entire distribution system must be fully flushed and cleaned so that impurities of the previously expired batch do not contaminate successive batch mixtures. Further, the remaining mixture 123 contained in the distribution system becomes a waste product that must be disposed of which is both inefficient and typically expensive.
Yet another problem with the batch process system 100 is that often the mixture 123 is hazardous (e.g., caustic, acidic, flammable, poisonous, etc.). Because the mixture 123 is hazardous, the pressurized batch mixing tank 110 and delivery tank 122 must be very closely monitored and controlled. Further, the batch-mixing tank 110 and delivery tank 122 are typically double walled to provide added safety containment of the hazardous mixture 123. The safety requirements of storing and pressurizing quantities of the hazardous mixture 123 increase the complexities of the batch process system 100 and the cost. Therefore the batch process system 100 is more expensive and less reliable than required.
Typically the batch process system 100 yields inconsistent batches because one batch is not exactly the same as another batch. Inconsistent batches often cause inconsistent CMP process results. The batches may be inconsistent because the measurements, such as the respective amounts of the first and second chemicals 101, 103, are different from one batch to another or because one batch has aged longer before use than another batch.
Similarly, the batch process system 100 does not produce a continuous and consistent mixture. This is because typical mixture control is in the batch mixing process in the batch-mixing tank 110. Once the mixture 123 is combined in the batch mixing tank 110 there typically is no further monitoring or testing to determine if the mixture is correct or becomes too aged or contaminated. As a result, if the mixture 123 becomes incorrect, then the CMP results could also become incorrect.
Another problem with most batch-type mixing systems is that a quantity of the mixture 123 is prepared in advance of the actual need of the mixture 123. If for any reason the mixture 123 is not needed (e.g., the CMP process is delayed until after the mixture 123 is too aged), then the entire mixture 123 must be discarded as a waste product. This results in excessive waste, which is both inefficient and typically expensive.
In view of the foregoing, there is a need for a more efficient, accurate delivery system of the CMP chemicals.